1. Field of the Invention
The present invention relates to an apparatus for and a method of concentrating solutions such as fruit juices, and more particularly to an apparatus for and a method of concentrating solutions with a membrane module which comprises a reverse osmosis membrane.
2. Description of the Relevant Art
There have heretofore been known concentrating apparatus which include membrane modules comprising reverse osmosis membranes. A solution to be concentrated is supplied into the membrane module. Then, a pressure higher than the osmotic pressure is applied to the solution in the membrane module to force pure water to pass from the solution through the membrane, thereby concentrating the solution.
The conventional concentrating apparatus with the reverse osmosis membrane have however suffered certain processing limitations. More specifically, the osmotic pressure (.pi.) of a solution is related to its concentration (C) as follows: EQU .pi.=1.43653.times.C+68.1016.times.C.sup.3
As can be seen from the above equation, the osmotic pressure of a solution is proportional to its concentration. Since a solution to be concentrated has to be subjected to a pressure higher than the osmotic pressure in the concentrating apparatus with the reverse osmosis membrane, it is necessary to employ a large-capacity pressure pump for applying a higher pressure if the solution has to be concentrated to a higher concentration. As the concentration of the solution becomes higher, the flux density of pure water flowing through the membrane is greatly reduced, resulting in a reduction in the concentrating efficiency. Therefore, there is a limitation on the concentration which can be achieved even if the large-capacity pressure pump is employed.